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A close-up of Elestor's advanced flow battery technology

The fundamentals

3D explanation of Elestor's flow battery

Working principle

Flow batteries were initially developed in the 1960s by The USA’s National Aeronautics and Space Administration, better known simply as NASA. But it wasn’t until the 1980s that their popularity picked up speed, after they were proven to last for more than 10,000 charge/discharge cycles. Along with the continuously growing installed base of renewable energy systems, most notably solar and wind power, it has become obvious that the need to store large, indeed very large, quantities of electrical energy for longer periods of time is growing equally quickly. Such energy storage is essential if we are to achieve a total transition from fossil fuels to renewable energy.

The term flow battery covers a family of storage systems where each one will apply the same fundamental working principle, while using different combinations of active materials. The heart of a flow battery is a so-called electrochemical cell, which is a multi-layer assembly of an ion-selective membrane, catalyst layers and electrodes.

A complete flow battery system, also referred to as a redox flow battery or RFB, is constructed around such electrochemical cells, where chemical energy is provided by the chemical reaction of two active materials. The active materials are contained within the system, separated by the membrane, and circulate in a closed loop, each one in their own respective space.

When an electrical power source is connected, which is when the battery is charging, a chemical redox reaction starts. Ion exchange then occurs through the membrane, resulting in electric current. During discharge, when applying an electrical load, the reverse chemical reaction takes place.

The voltage of the electrochemical cell is determined by the Nernst equation and ranges in practical applications from 1.0 to 2.2 V, depending on the selected active materials.

In order to increase the total electrical power, individual electrochemical cells are stacked, which is another way to say that the cells are electrically interconnected in series. To design systems with (very) large power levels, multiple stack assemblies can be interconnected.

3D close-up of Bromine-based chemistry driving Elestor's energy storage innovation

The power [MW] of a flow battery system, as depicted above, is determined by the surface area of the ion-selective membrane, while the capacity [MWh] of the system is determined by the volume of the catholyte and anolyte reservoirs.

The fact that the membrane surface area and the reservoir volumes can be dimensioned individually highlights one of the most distinguishing properties of flow batteries, as opposed to traditional electricity storage systems where power [MW] and capacity [MWh] scale simultaneously.

Dive deeper

The hydrogen-bromine flow battery for a large scale integration of variable renewable electricity: State-of-the-art review

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Y.A. Hugo, W. Kout, G. Dalessi

Elestor B.V., Utrechtseweg 310-H40, 6812 AR Arnhem, The Netherlands


This article presents a state-of-the-art review of the hydrogen-bromine battery technology. The review aims to elaborate on the following topics: (1) the hydrogen-bromine flow battery, (2) the current status of technical developments on short-term and long-term cycling, and (3) the future direction for technology development.

Performance mapping of cation exchange membranes for hydrogen-bromine flow batteries for energy storage

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Yohanes Antonius Hugo a, b, Wiebrand Kout b, Antoni Forner-Cuenca a, Zandrie Borneman a, c, Kitty Nijmeijer a, c, *
a Membrane Materials and Processes, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, PO Box 513, 5600MB Eindhoven, the Netherlands
b Elestor B.V., 6827 AV Arnhem, the Netherlands
c Dutch Institute for Fundamental Energy Research (DIFFER), P.O. Box 6336, 5600 HH Eindhoven, the Netherlands
⁎ Corresponding author. E-mail address: (K. Nijmeijer).


Electricity storage is essential for the transition to sustainable energy sources. Hydrogen-bromine flow batteries (HBFBs) are promising cost-effective energy storage systems. In HBFBs, proton exchange membranes are required to separate the two reactive materials, enabling proton transport for charge balancing. In this paper, we present a comprehensive overview of the key properties and an experimental performance map of cation exchange membranes for HBFBs. Our study shows that membrane water uptake is an important property due to its strong correlation with membrane resistance and bromide species crossover. Long chain perfluorosulfonic acid (LC PFSA) membranes are shown to have a better power density–crossover tradeoff and a higher stability than other types of functionalized membranes. The good power density-crossover tradeoff of LC PFSA membranes is the result of the high level of separation of hydrophobic and hydrophilic domains in the membrane, leading to well-connected ionic pathways for proton transport. Reinforcement of long chain LC PFSA membranes further improves their tradeoff because it mechanically constrains the swelling (lower water uptake), resulting in a lower crossover but a similar peak power density. Consequently, reinforced LC PFSA membranes are the most promising option for HBFBs.

Low-cost wire-electrospun sulfonated poly(ether ether ketone)/poly (vinylidene fluoride) blend membranes for hydrogen-bromine flow batteries

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Sanaz Abbasia, b, Antoni Forner-Cuencaa  Wiebrand Koutb, Kitty Nijmeijer a, c, Zandrie Borneman a, c, *
Membrane Materials and Processes, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, PO Box 513, 5600MB Eindhoven, the Netherlands
b Elestor B.V., 6827 AV Arnhem, the Netherlands
c Dutch Institute for Fundamental Energy Research (DIFFER), P.O. Box 6336, 5600 HH Eindhoven, the Netherlands
⁎ Corresponding author. Membrane Materials and Processes, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB, Eindhoven, the Netherlands. E-mail address: (Z. Borneman).


Cost-effective dense membranes were developed by blending proton-conductive sulfonated poly(ether ether ketone) (SPEEK) with inert, mechanically stable poly(vinylidene fluoride) (PVDF) for hydrogen-bromine flow batteries (HBFBs). Wire-electrospinning followed by hot-pressing was employed to prepare dense membranes. Their properties and performance were compared to solution-cast membranes of similar composition and thickness. Electrospinning improved the ionic conductivity and bromine diffusion properties by providing interconnected ion-conductive SPEEK nanofiber pathways through a PVDF matrix. Relatively thin (~50–60 μm) electrospun membranes with a SPEEK/PVDF ratio (wt%/wt%) of 90/10 and 80/20 showed comparable Br3 − diffusion rates as the relatively thick and commercially available perfluorosulfonic acid (PFSA) membrane (~100 μm) at a 35%–42% lower proton conductivity. The latter can be attributed to the poorer ion conductivity of SPEEK compared to PFSA and the presence of PVDF. The HBFB single cell featured the best polarization behavior and ohmic area resistance with the electrospun membrane containing 80/20 (wt%/wt%) SPEEK/PVDF. However, the low thickness and insufficient chemical/mechanical stability of the ES 80/20 causes a rapid decay in the HBFB cycling performance. This study promotes a life-time comparison study between the low-cost wire- electrospun SPEEK/PVDF blend membranes (~€100 m− 2) and the typically used PFSA membranes (~€400 m− 2) for a long-term HBFB performance.

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